Add a vocal warning to ath_hal_computetxtime() function is used for non-11n rates.
Add a vocal warning to ath_hal_computetxtime() function is used for non-11n rates.

It's used to calculate:

* the initial per-rate entries for short/long preamble ACK durations;
* packet durations for TDMA slot decisions;
* RTS/CTS protection durations;
* updating the duration field in the 802.11 frame header

This way invalid durations will generate a warning, prompting for it to be
fixed.

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